This invention relates to a process for the production of pure distilled isocyanates by reaction of the corresponding amines with phosgene in a suitable solvent and working up of the reaction product by multistage distillation.
The industrial production of distilled isocyanates by reaction of amines with phosgene in solvents is known and is described in detail in the literature (Ullmanns Encyklopadie der technischen Chemie, 4th Edition, Vol. 13, pages 347-357, Verlag Chemie GmbH, D-6940 weinheim, 1977). This disclosure also teaches that a stream of secondary product is obtained in the course of the production of pure distilled isocyanates. This stream of secondary product has to be disposed of as residue after the removal of free isocyanates by distillation. It is possible on a laboratory scale to remove considerable amounts of more free isocyanate from this stream by distillation. However, the residue remaining becomes a hard crosslinked mass which can no longer be handled in the industrial process. On a commercial production scale, therefore, around 20 to 40% free isocyanate is left in the residue stream to be disposed of to ensure that this stream can be handled. This results in the loss of valuable material and more waste to be disposed of.
Processes for recovering more free isocyanate from the residue are known. For example, GB-PS 1,408,745 describes an extraction process for recovering the free isocyanate. EP 269,218 describes a working up process in which the residue stream is heated with a bath of molten metal or metal salts. DE 2,915,830 describes a process for distilling the stream of residue in a fluidized bed. Each one of these processes requires expensive apparatus and/or auxiliaries. Auxiliaries such as metals or metal salts seriously complicate the subsequent disposal process.